Planar semiconductor devices including patterned thin film structures formed on a semiconductor substrate can generate a considerable amount of heat during normal operation. For example, a single emitter laser diode chip rated at 5 W of output radiation at an efficiency of 40%, will generate 3 W of heat during operation at full rated power. This heat needs to be removed to prevent overheating and failure of the device.
Heat removal is achieved by mounting a semiconductor device onto a heat sink made of a material having a good thermal conductivity, e.g. copper. Once the heat is transferred to the copper heat sink, it can be removed by cooling the heat sink with, for example, a Peltier cooler. It is therefore essential to ensure a good thermal contact between a semiconductor device and a heat sink. In order to provide such a contact, and also to ensure that the device is mounted in a reliable and stable fashion, soldering is often used.
Soldering a semiconductor device, especially a radiation-emitting semiconductor device such as a laser diode, to a copper heat sink has a serious drawback. A laser diode chip has to be mounted at a certain position relative to a collimating lens. A shift of the chip relative to the lens degrades a laser performance and should be avoided. To avoid creeping of laser diode chip during operation or storage at varying temperatures, a hard solder is frequently used. However, soldering of a semiconductor device on a GaAs or a silicon substrate, having relatively small coefficient of thermal expansion, to copper having a large coefficient of thermal expansion, with a hard solder creates a significant amount of residual stress in the semiconductor device, which greatly reduces reliability of the latter. A material with a coefficient of thermal expansion matching that of the semiconductor device can be used to reduce the stress to a low enough value. For example, copper-tungsten (CuW) alloy is used in prior art to match the coefficient of thermal expansion of gallium arsenide (GaAs). However, thermally matching alloys are often expensive to make and difficult to process.
An alternative approach established in the prior art is based on employing a submount, placed between a semiconductor device and a heat sink, for better thermal matching between the semiconductor device and the surface it is mounted on. To reduce the residual stress in a semiconductor device mounted on a submount, Mochida et al., in US20050127144A1 which is incorporated herein by reference, describe various techniques using pressure bonding and, or creating temperature gradients, to offset the residual stresses generated upon cooling the compound heat sink. Further, Moriya et al., in U.S. Pat. No. 6,961,357 which is incorporated herein by reference, optimize a shape of a submount to reduce residual stress at a particular point, corresponding to the light emitting region of a mounted laser diode chip, to a value lower than 20 MPa, which is considered in U.S. Pat. No. 6,961,357 to be a threshold value of stress above which a defect rate increases considerably (see FIG. 5 of said Patent document). Still yet further, Yamane et al., in US20040201029A1 which is incorporated herein by reference, describe various methods of applying soldering compounds aiming at minimizing the melting point of compound solder films, to lower soldering temperature, and to lower the residual mechanical stresses correspondingly.
The abovementioned approaches utilizing a submount for relieving the stress in a semiconductor device share common problems. Mounting methods employing a submount require an extra process step of mounting the submount on a heat sink, or mounting the semiconductor device to the submount, whichever step is done first. A special mounting equipment needs to be developed, for example, in case of US20050127144A1 by Mochida et al., a heated mounting chuck (collet) needed to be developed. More complicated mounting process and utilization of special materials for precise thermal matching of submount to the semiconductor device increase manufacturing time and cost, as compared to mounting of semiconductor devices directly onto a copper heat sink.
It is therefore an object of the present invention to provide a method for mounting a semiconductor device allowing one to considerably lower the levels of residual mechanical stress in a mounted semiconductor device without having to use expensive or difficult to machine submount materials or adding new major steps in the manufacturing process. Further, it is an object of the present invention to provide a mounted semiconductor device having low levels of the residual mechanical stress, lower than 20 MPa and, preferably, lower than 10 MPa.
The device and method of present invention meet the above stated objectives. Not only that, but the method of the present invention allows one to use a hard solder having relatively high melting point, to ensure a high mechanical stability and reliability of a mounted device.